Because you can't really tax water.
This just stinks of SCAM.
Be very suspicious. It's too good to be true, and what do we usually think when something looks too good to be true? What would we say if a politician said we were going to have cars that ran on water? You wouldn't be able to hear for people calling bullshit. The energy to break the water down into H and O isn't going to come out of thin air, and whatever they're using to provide that energy could have been used to make the wheels go round in the first place. No such thing as a free lunch I'm afraid.